1. Field of the Invention
The invention relates to HF communications and in particular to a multichannel frequency diversity DPSK communications system for the HF radio band.
2. Discussion of Prior Art
The purpose of frequency diversity in a communications system is to overcome the vagaries of long range HF radio propagation and interference and thereby improve the ability to reliably detect the transmitted signal with greatly reduced errors and with increased availability.
If transmitted signals are sent using a plurality of different radio frequencies the intended receiver will be able to exploit the diversity reception to:
a. reduce the received bit error rate PA1 b. avoid co-channel interference from other radio transmissions; PA1 c. overcome multipath time dispersion; PA1 d. overcome channel fading; PA1 e. reduce the effects of time/diurnal variations in propagation; PA1 f. exploit sporadic and transitory propagation; PA1 g. operate with lower transmitter powers: PA1 h. have improved performance (greater data rates); have increased availability (on-demand communications).
Diversity reception requires the provision of two or more (K) transmitted signals, each containing the same message (either simultaneously or time interleaved). On different radio frequency carriers having advantageously uncorrelated propagation characteristics: each carrier frequency defining a diversity channel.
At the receiver the diversity channels must be properly recombined in order to ideally produce an output signal which will have a much lower combined BER (bit error rate) than in any one received channel. In the simplest diversity combined the channel with the best S/N (signal-to-noise ratio) or lowest BER will be switched to the output. This type of switch `combining` only works well, however, when at least one channel is always good. When the S/N is simultaneously poor in all the channels the output will be also be poor. A more advantageous method of diversity combining is to sum the received branches after weighting each channel. The channels can be weighted according to their S/N, for example; such systems are known as Maximal Ratio Combining. Using this technique it is possible to coherently combine the wanted signals (if channel co-phasing can be used) whilst at the same time only adding the noise in each channel incoherently. This produces a combined S/N which will be 10 Log(K)dBs better than any individual diversity channel, where the S/N is the same in each. However, to be most effective at HF, the channel frequencies must be separated by more than the correlation bandwidth (the range of frequencies over which noise signals are correlated). This will ensure each channel path will be totally uncorrelated in propagation characteristics such as, fading and multipath as well as interference. Unfortunately, this also means the phase characteristics for each path will also be very different from baud to baud. This uncorrelated phasing characteristic between channels will make it very difficult to properly co-phase the wanted signals from each path particularly since the S/N will normally be poor in each. At HF therefore, diversity combining can normally only be achieved using noncoherent signal combining.